B lymphocytes and the antibodies they produce play a critical role in host defense. Cell fate diversification of activated lymphocytes is essential to achieve the regenerative state required for long-lived immunity. The mechanisms responsible for generating B cell fate diversity, however, are poorly understood. Substantial preliminary data suggest B cells can diversify the fates and functions of their daughter cells using an evolutionarily conserved strategy to allocate unequal amounts of key components. This project will test whether asymmetric cell division is a cardinal feature of the B cell- mediated immune response. The proposal develops novel methodologies to image the characteristics of dividing B cells during the course of an immune response. The aims of the project are to test the importance of unequal cellular inheritance of three different transcriptional regulators of B cells Bcl-6, Pax5, and T-bet, each of which is known to regulate critical cell fate decisions. The experiments will test the hypotheses that asymmetric division is iteratively used to meet the opposing demands of terminal differentiation and self-renewal, as well to refine and diversify the functional properties of antibodies by regulating germinal center entry and class switch choice. This project will also examine whether ancestral regulators of cell polarity are responsible for establishing asymmetric B cell division, and how asymmetrically inherited proteins could mediate fate disparity in daughter B cells. These studies should provide a framework for rational engineering of immune responses and vaccines against microbial agents. This project should also address fundamental uncertainties regarding the principle of clonal selection of lymphocytes in response to infectious diseases or during situations when our immune cells attack our own selves.